1. Field of the Invention
This invention relates to distributed Bragg reflector (DBR) lasers and, in particular, to DBR lasers having a quarter-wavelength (xcex/4) phase shift section for improved single-longitudinal mode operation thereof.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
There are several types of lasers, including gas lasers, solid-state lasers, liquid (dye) lasers, free electron, and semiconductor lasers. All lasers have a laser cavity defined by at least two laser cavity mirrors, and an optical gain medium in the laser cavity. The gain medium amplifies electromagnetic waves (light) in the cavity by stimulated emission, thereby providing optical gain.
In semiconductor lasers, a semiconductor active region serves as the gain medium. Semiconductor lasers may be diode (bipolar) lasers or non-diode, unipolar lasers such as quantum cascade (QC) lasers. Semiconductor lasers are used for a variety of industrial and scientific applications and can be built with a variety of structures and semiconductor materials.
The use of semiconductor lasers for forming a source of optical energy is attractive for a number of reasons. Semiconductor lasers have a relatively small volume and consume a small amount of power as compared to conventional laser devices. Further, semiconductor lasers can be fabricated as monolithic devices, which do not require a combination of a resonant cavity with external mirrors and other structures to generate a coherent output laser beam.
The optical gain of a laser is a measure of how well a gain medium such as an active region amplifies photons by stimulated emission. The primary function of the active region in a semiconductor laser is to provide sufficient laser gain to permit lasing to occur. The active region may employ various materials and structures to provide a suitable collection of atoms or molecules capable of undergoing stimulated emission at a given lasing wavelength, so as to amplify light at this wavelength. The active region may comprise, for example, a superlattice structure, or a single- or multiple quantum well (MQW) structure.
Amplification by stimulated emission in the active region of a semiconductor laser is described as follows. The semiconductor active region contains some electrons at a higher, excited state or energy level, and some at a lower, resting (ground) state or energy level. The number and percentage of excited electrons can be increased by pumping the active region with a pumping energy, from some energy source such as an electrical current or optical pump. Excited electrons spontaneously fall to a lower state, xe2x80x9crecombiningxe2x80x9d with a hole. The recombination may be either radiative or non-radiative. When radiative recombination occurs, a photon is emitted with the same energy as the difference in energy between the hole and electron energy states.
Stimulated emission, as opposed to spontaneous emission, occurs when radiative recombination of an electron-hole pair is stimulated by interaction with a photon. In particular, stimulated emission occurs when a photon with an energy equal to the difference between an electron""s energy and a lower energy interacts with the electron. In this case, the photon stimulates the electron to fall into the lower energy state, thereby emitting a second photon. The second photon has the unique property that it has the same energy, frequency, and phase as the original photon. Thus, when the photons produced by spontaneous (or stimulated) emission interact with other high energy state electrons, stimulated emission can occur so that two photons with identical characteristics are present. (Viewed as waves, the atom emits a wave having twice the amplitude as that of the original photon interacting with the atom.) I.e., one photon of a given. energy, frequency, and phase produces a second photon of the same energy, frequency, and phase; and these two photons may each, if not absorbed, stimulate further photon emissions, some of which can themselves stimulate further emissions, and so on.
Amplification by stimulated emission requires that more photons are produced by stimulated emission than are absorbed by lower-state electrons. This condition, known as population inversion, occurs when there are more excited (upper lasing level) electrons than ground-state (lower lasing level) electrons. If there were more lower state than upper state electrons, then more photons would be absorbed by the lower energy electrons (causing upward excitations) than would be produced by stimulated emission. When there is a population inversion, however, enough electrons are in the excited state so as to prevent absorption by ground-state electrons from sabotaging the amplification process. Thus, when population inversion is achieved, stimulated emission predominates over stimulated absorption, thus producing amplication of light (optical gain). If there is population inversion, lasing is therefore possible, if other necessary conditions are also present.
Population inversion is achieved by applying a sufficient pumping energy to the active region, to raise a sufficient number of electrons to the excited state. Various forms of pumping energy may be utilized to excite electrons in the active region and to achieve population inversion and lasing. For example, semiconductor lasers of various types may be electrically pumped (EP), by a DC or alternating current. Optical pumping (OP) or other pumping methods, such as electron beam pumping, may also be used. EP semiconductor lasers are typically powered by applying an electrical potential difference across the active region, which causes a current to flow therein. As a result of the potential applied, charge carriers (electrons and holes) are injected from opposite directions into an active region. This gives rise to an increase in spontaneous generation of photons, and also increases the number of excited state electrons so as to achieve population inversion.
In a semiconductor laser, an active region is sandwiched between the cavity mirrors, and pumped with a pumping energy to cause population inversion. Photons are spontaneously emitted in the active region. Some of those photons travel in a direction perpendicular to the reflectors of the laser cavity. As a result of the ensuing reflections, the photons travel through the active region multiple times, being amplified by stimulated emission on each pass through the active region. Thus, photons reflecting in the cavity experience gain when they pass through the active region. However, loss is also experienced in the cavity, for example by extraction of the output laser beam, which can be about 1% of the coherent cavity light, by absorption or scattering caused by less than perfect (100%) reflectance (reflectivity) of the cavity mirrors, and other causes of loss.
Therefore, for lasing to occur, there must be not only gain (amplification by stimulated emission) in the active region, but enough gain to overcome all losses in the laser cavity as well as allow an output beam to be extracted, while still allowing laser action to continue. Gain is a function of wavelength. The minimum gain that will permit lasing, given the cavity losses, for a given wavelength or wavelength range, is the threshold lasing gain of the laser medium for that wavelength or range. A given wavelength is associated with a given threshold gain, and may be characterized by that threshold gain, for a given laser structure. (For EP lasers, the lowest drive current level at which the output of the laser results primarily from stimulated emission rather than spontaneous emission is referred to as the lasing threshold current.)
When the active region provides the threshold lasing gain over a given wavelength range, there will be a sufficient amount of radiative recombinations stimulated by photons, so that the number of photons traveling between the reflectors tends to increase, giving rise to amplification of light and lasing. This causes coherent light to build up in the resonant cavity formed by the two mirrors, a portion of which passes through one of the mirrors (the xe2x80x9cexitxe2x80x9d mirror) as the output laser beam.
Because a coherent beam makes multiple passes through the optical cavity, an interference-induced longitudinal mode structure or wave is observed. The wave along the laser cavity is a standing EM wave and the cavity of effective optical length L only resonates when the effective optical path difference between the reflected wavefronts is an integral number of whole wavelengths (the effective cavity length or optical path difference takes phase-shifting effects at the mirrors into account). In other words, lasing is only possible at wavelengths for which the round-trip phase is a multiple of 2xcfx80. The set of possible wavelengths that satisfy the standing wave condition is termed the set of longitudinal modes of the cavity. Although there are an infinite number of such wavelengths, only a finite number of these fall within the wavelength range over which the gain spectrum of the active region exceeds the threshold lasing gain. The laser will lase only at one or more of the possible longitudinal (wavelength) modes which fit into this wavelength range.
Semiconductor lasers may be edge-emitting lasers or surface-emitting lasers (SELs). Edge-emitting semiconductor lasers output their radiation parallel to the wafer surface, in contrast to SELs, in which the radiation output is perpendicular to the wafer surface, as the name implies. In conventional Fabry-Perot (FP) edge-emitting lasers, a cleaved facet mirror is used to obtain the feedback for laser oscillation. In an FP laser, the two facets of the diode, the rear and the front (emitting) surface, are cleaved to establish the dimensions of the structure such that a primary longitudinal mode of resonance will exist at the desired wavelength.
Other semiconductor lasers, such as distributed-feedback (DFB) and distributed-Bragg reflector (DBR) lasers, employ one or more diffraction gratings to provide reflectance. The diffraction grating, also known as a Bragg grating, contains grating rows, stripes or xe2x80x9cteethxe2x80x9d arranged in a regular pitch for distributively feeding light back by Bragg reflection. Such devices provide feedback for lasing as a result of backward Bragg scattering from periodic variations of refractive index, instead of using conventional facets or mirrors to provide reflection.
A DFB laser typically has a uniform diffraction grating constructed in the waveguide itself, adjacent to (i.e., above or below) the active region layer. In such a laser, optical feedback is provided along the laser""s cavity length by means of a diffraction grating whose stripes or teeth run perpendicular to the length (longitudinal direction) of the laser cavity. The grating serves as an internal periodic feedback structure that establishes the wavelength of operation.
The term xe2x80x9cDBR laserxe2x80x9d typically denotes an edge-emitting laser comprising a grating that is not uniform, i.e. a grating that is not continuous over the entire longitudinal cavity. A typical is DBR laser has one or two grating sections fabricated in a waveguide external to the active region, which grating sections provide part or all of the reflectance of the cavity mirrors. For example, the DBR laser typically is an edge-emitting laser having two separate DBRs or gratings, each at either end of, and outside, the active region layer. The DBRs reflect and thus provide feedback at the desired wavelength.
Diffraction gratings provide optical feedback by the period variation of the effective refractive index of the grating. The period index variation causes a wavelength-selective feedback. Thus, the diffraction gratings of DFB and DBR lasers have a reflectance that is a function of wavelength. Maximum reflectance of a diffraction grating occurs around its so-called Bragg wavelength (xcexB), which is given by Eq. (1) below:
xcexB=2xcex9nexe2x80x83xe2x80x83(1)
where xcex9 is the period (pitch) of the grating and ne is the effective refractive index of the waveguide having the grating. Thus, the Bragg wavelength xcexB is determined by the grating pitch or period. The period of DFB and DBR gratings is typically selected so that the Bragg wavelength xcexB is equal to the desired operating wavelength of the laser. Diffraction gratings therefore give significant reflection, and thus significant optical feedback, only around the operating wavelength.
Single-wavelength (single longitudinal mode) operation is desirable for many applications, such as in high-bit-rate optical fiber communication. Single-mode operation refers to laser operation in which the intensity of the most-intense, or xe2x80x9cprimary,xe2x80x9d mode lasing is substantially greater than all other modes, including the next-most-intense and adjacent modes. Single-mode operation may be said to occur when the side mode suppression is at least a minimum amount (e.g., 30 dB), for example, the primary mode is at least 30 dB greater than its side modes (and all other modes). In telecommunications applications, for example, it is desirable that the laser emit at a single lasing wavelength at 1.31 xcexcm (and other closely spaced wavelengths), or at telecommunications wavelengths specified by the ITU grid, such as lasing wavelengths of 1.55 xcexcm (and other closely spaced wavelengths). These wavelength ranges are often used for telecommunications purposes because the loss of silica fibers is comparatively low at these wavelengths.
In a conventional DFB laser, an additional phase shift xcfx80 is introduced at the Bragg wavelength xcexB. Therefore, the round-trip phase at the Bragg wavelength xcexB is xcfx80+n2xcfx80, where n is an integer. However, xcfx80+n2xcfx80 is not a multiple of 2xcfx80. Thus, the phase condition for oscillation cannot be met at the Bragg wavelength xcexB (or corresponding Bragg frequency fB), and no mode occurs at this wavelength. Instead, there are two equally xe2x80x9cprimaryxe2x80x9d modes, at two frequencies slightly removed by some frequency f from the Bragg frequency fB, i.e. two optical modes having frequencies fBxc2x1f are present. This means that there are two modes having approximately equal threshold gain. These two modes are sometimes referred to as the right Bragg mode and the left Bragg mode.
The presence of these two xe2x80x9cco-primaryxe2x80x9d modes can give rise to an unpredictable lasing frequency, since oscillation often occurs, unpredictably, in either one of the modes, as these two so-called degenerate modes compete substantially equally for lasing as the dominant mode. Mode-hopping can also occur from one mode to the other. The two modes are thus problematic for wideband applications, since a single, narrow linewidth output is needed for wideband applications. Two modes having similar threshold gain are sometimes known as degenerate modes. The existence of two degenerate modes results in multi-mode operation, unpredictable modes, or mode hopping, sometimes referred to as mode degeneracy.
It is preferable to construct a laser in which there is only a single primary mode, i.e. in which there is no mode degeneracy. The insertion of a xcfx80/2 optical phase shift section into the DFB structure is one way to xe2x80x9csuppressxe2x80x9d or xe2x80x9cbreakxe2x80x9d the mode degeneracy, so as to achieve single-mode operation. This phase shift region creates an extra xcfx80/2 phase shift for each wave passing along it of wavelength xcexB. The xcfx80/2 phase shift section thus provides an extra round-trip phase shift of xcfx80, which adds to the additional phase shift n introduced by a conventional DFB laser at the Bragg wavelength xcexB, so that the round-trip phase is a multiple of 2xcfx80 at the Bragg wavelength xcexB. That is, xcfx80+(xcfx80+n2xcfx80)=2xcfx80+n2xcfx80=(n+1)2xcfx80, which is an integer multiple of 2xcfx80, since (n+1) is an integer.
Thus, with a xcfx80/2 phase shift section in the grating, the phase condition for oscillation can be met at the Bragg wavelength xcexB (or Bragg frequency fB). In effect, the xcfx80/2 phase shift technique provides for optical phase matching to adjust the main mode at the Bragg wavelength. This xe2x80x9creducesxe2x80x9d the number of resonance modes to one, and its resonance frequency coincides with the Bragg frequency fB. Thus, the xcfx80/2 phase shift section effectively provides lasing predominantly on a preferred and fixed Bragg mode. The result is a laser that has only one primary mode, i.e. only one longitudinal mode with the lowest threshold gain at the Bragg wavelength, thereby achieving stable single mode oscillation at the Bragg wavelength. (Conventional DBR lasers typically employ an actively controlled phase-shift section in which current is injected to control the phase shift.)
The xcfx80/2 phase shift of this technique is also known as a quarter-lambda, quarter-wavelength, or xcex/4 phase shift. This is because introducing a xcfx80/2 phase shift at xcexB is equivalent to adding a section of length xcex9/2=xcexB/(4ne) into the grating structure. Although xcex4 is literally a length, not a phase shift, the terminology xe2x80x9cxcex/4 phase shiftxe2x80x9d will be employed herein due to the conventional use of this term. It will be understood that a xe2x80x9cxcex/4 phase shiftxe2x80x9d, as the term is used in this application, refers to a phase shift of xcfx80/2, at wavelength xcexB, which phase shift is equivalent to the phase shift that would be obtained if a section of length xcex9/2 =xcexB/(4ne) were to be inserted into the optical path, i.e. the phase shift (at xcexB) resulting from increasing the optical path by length xcexB/4.
Various approaches have been employed to shift the optical phase by xcfx80/2 in DFB lasers. For example, to achieve a xcfx80/2 shift, many commercial lasers simply insert lithographically a physical xcex/4 phase shift into the grating mask. The resultant devices are known as xcex/4 or xcfx80/2 phase-shifted DFB lasers and oscillate with a single frequency close to the Bragg frequency of the grating. Such structures employing directly phase-shifted gratings are described in xe2x80x9cStability in Single Longitudinal Mode Operation in GaInAsP/InP Phase-Adjusted DFB Lasers,xe2x80x9d by Haruhisa Soda et al., IEEE J. Quantum Electronics, vol. QE-23, No. 6, June 1987, pp. 804-814 (Haruhisa Soda Reference), xe2x80x9cAsymmetric xcex/4-Shifted InGaAsP/InP DFB Lasers,xe2x80x9d Masashi Usami et al., IEEE J. Quantum Electronics, vol. QE-23, No. 6, June 1987, pp. 815-821 (Masashi Usami Reference); Distributed Feedback Semiconductor Lasers, by John Carroll, James Whiteway and Dick Plumb (London: Institution of Electrical Engineers, 1998) (Carroll Reference), section 1.7.2 (pp. 26-28); and Handbook of Distributed Feedback Laser Diodes, by Geert Morthier and Patrick Vankwikelberge (Boston: Artech House, Inc., 1997), section 4.1.4, pages 102-104 (Morthier and Vankwikelberge Reference).
Other techniques for shifting the optical phase by xcfx80/2 include: employing a nonuniform waveguide structure, as described in the Haruhisa Soda Reference; providing a waveguide having two straight portions and a bending portion, so that the longer bending portion causes a quarter-lambda shift, as described in U.S. Pat. No. 4,833,687; changing the thickness of the active layer in a phase shift section, as described in U.S. Pat. No. 4,847,856; and moving the left and right sections of the grating with respect to each other in a direction perpendicular to the longitudinal axis of the active region, as described in U.S. Pat. No. 5,052,015.
However, there are various disadvantages with conventional xcex/4 phase-shifting techniques. For example, directly inserting a physical xcex/4 phase shift into the grating mask can be difficult to manufacture, because it requires multiple grating fabrication steps, for example. Also, such phase-shift sections inserted directly into the corrugation of the grating does not give optimum dynamic wavelength stability during modulation, requiring more complex phase-adjusting techniques such as insertion of two xcex/8 phase shift sections, which can add to the difficulties of manufacture.
Further details of DFB lasers and diffraction gratings may be found in the Carroll Reference and in the Morthier and Vankwikelberge Reference.